1. Field of the Invention.
The presention invention relates generally to specialty seals, and more particularly, to so-called bell seal assemblies forming parts of high pressure steam turbines used, for example, in generating electricity.
2. Description of the Prior Art.
While steam turbines of the kind used by electric utilities, for example, have been in use for a number of years and are recognized as a highly satisfactory method of power generation, the construction and arrangement of many turbines is such that they present difficult sealing problems in particular areas.
A typical steam turbine, wherein the high pressure section rotor is disposed within so-called inner and outer cylinders requires steam to pass without leakage between the cylinders. This requires a static seal which will withstand extremely high pressures, high temperatures, and differential thermal expansion; the seal must be substantially fluid-tight and remain stable under conditions of extremely high velocity, sometimes pulsating steam flow. Dynamic instability, vibration, and thermal shock are repeatedly encountered in use by bell seal assemblies. The present invention is directed to an improvement in seals of this type; one prior art version of such a bell seal is shown in U.S. Pat. No. 3,907,308.
By way of further background, a typical steam turbine unit includes a rotor assembly journaled for rotation about a given axis and surrounded by so-called inner and outer cylinders. The inner cylinder includes, among other parts, a blade carrier ring which forms a part of the turbine stator and several nozzle chamber units each welded to the inner cylinder so as to become an integral part thereof. The outer cylinder includes a high pressure steam exhaust outlet, and a number of so-called inlet sleeve units, each of which extends inwardly in telescoping relation to its associated nozzle chamber in the inner cylinder.
From each of the several control valves (not shown), the steam enters the high-pressure turbine through an inlet sleeve integrally attached to the outer cylinder, into a nozzle chamber integrally attached to the inner cylinder, and then passes through the nozzles and rotating blades of the control stage. At this point, the steam from the several parallel inlet paths merges together and then flows through the rest of the high-pressure turbine blading comprised of an array of stationary and rotating blade rows.
The bell seal assembly forms the connection between the inlet sleeve and its associated nozzle chamber, accommodating some relative motion and misalignment between them while simultaneously sealing against leakage of the supply steam entering the turbine into the space between the outer and inner cylinders which contains steam at a considerably lower pressure.
Because of the nature of the fluid flow, in a typical case being steam at pressures ranging from 2400 to 3500 p.s.i., generally at temperatures of 1000.degree. F., there are problems of thermal shock, thermal expansion, and manufacturing alignment tolerances, which mandate that a highly versatile seal be provided. Further, the seal must also remain stable under conditions of pulsating pressure, and must resist vibration, wear and loss of effective sealing contact in use.
In many prior art sealing application, a bell type seal has generally been settled upon as standard. The characteristic bell or flanged cylinder shape of the seal is such that it may be positioned and retained relative to the inlet sleeve forming a part of the outer cylinder by a special retaining nut which in turn allows the seal the predetermined amount of "float", or free but limited movement, which is required for proper centering and sealing action.
In the upper half of the turbine cylinder, the flange of the bell seal unit is retained by an upwardly directed shoulder surface portion of the retaining nut and a lower end face sealing surface on the inlet sleeve. The skirt of the bell seal extends in a downstream direction and enters a skirt-receiving annular groove formed in the nozzle chamber. In use, high pressure in the inlet sleeve passage serves to move the bell unit slightly axially upstream into the end face sealing relation just described. The lower margin of the skirt expands thermally into a generally fluid-tight contact with an oppositely directed cylindrical sealing surface of the nozzle chamber annulus.
While this general arrangement has been known and is considered perhaps the most satisfactory, like all other difficult sealing installations, it is considered capable of still further improvement. Because of the conditions under which the turbine is operated, the seal assembly is exposed to severe temperature diffentials, extreme pressure gradients and rapid pressure fluctuations. When these forces cause leakage around the seal periphery, transient pressures cause misalignment, noise, chatter and, very often, a resonant movement of the parts relative to each other in the seal region. These problems can cause accelerated sealing failure, particularly where the surfaces are damaged by the resulting vibration. In some cases, vibration is so extreme that loud noise is created, and abrasion of the bell skirt and the nozzle chamber wall results. Ultimately, this may result in cracks and fissures in the bell seal or elsewhere in the inlet sleeve or the nozzle chamber.
Unfortunately, however, the clearances allowing this movement may not simply be eliminated. The bell must be free to move axially to create the proper end face seal, to move radially for alignment, and to expand radially to create the peripheral side wall seal which will withstand the extreme pressures and thermal gradients referred to. During shutdowns, after extended use for maintenance or otherwise, the parts must "shrink" to a thermally relaxed condition which will provide sufficient clearance for the inner and outer cylinders to be removed from each other without damage. These seals or other components may then be replaced and re-installed as needed.
In view of the failure of the prior art to provide a completely satisfactory seal, it is a general object of the present invention to provide an improved bell seal assembly.
Another object of the invention is to provide a bell seal assembly using a bell seal having a particular arrangement of lands and grooves which isolate the guiding and locating function from the wall-to-skirt primary sealing function so both of these functions can be better achieved and maintained.
A further object of the invention is to provide a seal which is capable of improved performance without requiring materials which are more expensive or difficult to work with than those presently in use.
A still further object of the invention is to provide a seal which will provide improved functioning without measurably increased cost of manufacture.
Another object of the invention is to provide a bell seal having a retaining nut unit with a novel support arrangement for the bell seal flange.
A still further object of the invention is to provide a combination retaining nut and bell seal wherein the retaining nut includes a flange seal support element having a plurality of slots extending axially from its lower flange end surface to its flange support surface to provide improved pressure equalization on the flange of the bell seal, and also to provide engagement surfaces for the nut adjusting tool used to position the bell unit.
A further object of the invention is to provide a seal assembly which includes a bell having a skirt portion with its lower margin subdivided into a labyrinth seal region of land and groove configuration and a guiding and locating region having guide lands and grooves extending axially between the lower skirt end portion and an area adjacent the labyrinth seal region, which is in turn comprised of a plurality of circumferentially extending, sidewall engaging lands spaced apart by circumferential grooves.
A still further object of the invention is to provide a bell seal assembly wherein portions of the retaining nut and the nozzle chamber are shaped to provide a seal cavity of a desired contour between them for improved dynamic flow and reduced excitation caused by steam pressure variations in the inlet sleeve and the seal cavity.
Yet another object is to provide a seal having a retaining nut with a smooth, inner surface and an uninterrupted lower edge, which is smoothly contoured for reduced cavity turbulence in use.
These and other objects and advantages of the invention are achieved and practiced by providing a bell seal assembly having a bell unit with a radially extending flange having an upwardly directed end face sealing surface and a downwardly directed flange support surface, a skirt portion with a lower margin subdivided into a labyrinth seal region and a grooved guide surface region, with the retaining nut including a support flange having axial grooves in its outer diameter to provide a fluid flow path between the downstream end face of the nut flange and the support surface of the bell flange. The manner in which these and other objects and advantages of the invention are achieved in practice will become more clearly apparent when reference is made to the following detailed description of the preferred embodiment of the invention set forth by way of example, and shown in the accompanying drawings, and where like numbers indicate corresponding parts throughout the several figures.